In a Q&A, architect Thom Mayne, winner of the Pritzker Prize, talks about green design.

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San Francisco's coolest building? It's his. Architect Thom Mayne is a maverick. His buildings are edgy: Elevators skip stops to force people to take the stairs. Photovoltaic walls convert sunlight into electricity. In 2005, Mayne accepted his profession's highest honor, the Pritzker Prize. And now his design for San Francisco's new Federal Building—an 18-story concrete slab draped in perforated gray steel—is turning heads on the street.

The structure actually breathes. Air circulates between its floors and between its steel outer cloak and glass inner skin, while a weather-sensitive computer opens and shuts its windows. The building, at 90 Seventh Street, is open to visitors.

Q What part of the design are you most proud of?A The atrium, where I've created a big interior public space. This used to be done all the time—going back to the Parthenon in Greece—as a symbol that a building was of civic importance.

Q Your intentions for the sky garden?A I really wanted to create spaces where people would feel comfortable. I want you to sit in the garden and read a book or come on your lunch break. This is one of the world's most beautiful cities, and the garden is an incredibly magical environment up in the air.

Q How did you know that green design would be hot when you started the job in 1999?A My company has an office in Europe, where there are different environmental standards. I felt it was time for us here to match aesthetics with performance.

Q What will visitors take from the place?A I hope it changes their idea of what government buildings can be. Whatever you think about the politics of the moment, our buildings should honor what we are as a culture—this is us, this is me. You don't have to like the building, but you should sense that it's important.

Q How would you respond to people who think that concrete is grim, that granite or brick would be more appealing?A To some extent, I'd agree with them. I love tough modern architecture, but concrete is a poor man's stone. We built this for $240 a square foot. That's one-third the cost of the de Young Museum (by Swiss architects Jacques Herzog and Pierre de Meuron, which opened in San Francisco in 2005). Give us another $500 a square foot, and it's a different building.

Q But overall you're pleased?A Absolutely. There's a beautiful lobby, there's a cafe building framing the plaza where you can sit outside and meet people. The Turrell is great (Arizona artist James Turrell did a multi-colored lighting installation that slices from the sky garden to the plaza and glows at night) and I love the Ruschas (Los Angeles artist Ed Ruscha contributed large prints for a series of tall lounges sprinkled through the tower).

Q Why put a large plaza in a dicey part of the city?A One reason is the presence of the courthouse across the street (a beaux-arts structure from 1905 with a classical exterior, marble-lined corridors and vaulted tile ceilings). It's an important building politically and culturally and architecturally. I wanted to allow that landmark to breathe, to be seen properly. Also, a plaza seems logical in this part of the city. This is the next vibrant area for redevelopment. I think the plaza is going to become a social place. That's why we made sure to have the café building at the corner.

Q Why nudge people to use stairs?A To make them healthy! You can't read a paper without seeing a study every other day about our culture's problems of obesity. To walk up and down a stairway seems logical.

Q Some workers have commented about having to use stairways and about the amount of light that comes through the southwest-facing windows.A We should be handing out manuals, helping verse them in how the building works. It's important that they feel connected to the broader ethic of the building.

Q You started working on this design in 1999. Why did you focus on sustainability? The issue wasn't nearly as big back then.A A lot of the things I did here seem so obvious. It's not air conditioning or electrical systems, it's about shapes—thin buildings can have natural light and air. And with that comes a more human scale. The different set of standards in Europe gave me an increased awareness of our influence over the environment in terms of the impact buildings can have. Sustainability is an incredibly important and prominent issue right now, but it was certainly relevant back then.

Q Do you consider yourself a green architect?A I'm absolutely not a green architect. To me that means hideous solar panels on roofs that destroy a community's aesthetics. I'm interested in showing how the two go together, aesthetics and the environment.

Q How was it having the federal government as a client?A It's been great. It's been lovely. And it's been very personal—instead of a big autonomous client or some amorphous bureaucracy, I've felt extremely connected to the individuals I was working with. This is a risky building in a lot of ways, but they gave me nothing but support.

Q What was it like designing a major structure in San Francisco?A San Francisco is such an odd city. The buildings aren't that interesting—it's not a Paris or a London—but the totality is so beautiful.